I am a mercedes benz master tech I have been repairing MB for 30 years now and this car is like a tank with proper maintance and care it will run a very long time hundreds and hundreds of thousands of kilometers, however you mention it runs rough it is possible that one of the injectors is not injecting properly, it would be wise to have them tested or replaced, also at the same time a compression test done, the only other thing people tend to overlook on these enginnes with high mileage (200000) is replacing the timing chain, it is not a long procedure and any mercedes tech should be able to do, its just a matter of removing the valve cover attaching the new chain to the old and turning the engine in proper direction by hand, there is a special tool for riviting the chain link together and another tool that covers the camshaft wheel to keep the chain in constance mesh with the wheel, after it is installed the injection timing must be rechecked, the cost of doing this is very cheap compared to having the chain break and damaging the engine.